Pressure and temperature are important parameters to be determined in logging an oilfield earth formation. It is known that both can be measured using fiber optic grating based strain sensors.
Eric Udd, in U.S. Pat. No. 5,828,059 (“the Udd patent”), discloses a system and method to sense the application of transverse stress to an optic fiber having fiber optic gratings. The system includes a light source that produces a relatively wide spectrum light beam. The light beam is reflected or transmitted off of an optical grating in the core of an optical fiber that is transversely stressed either directly or by the exposure to pressure when the fiber is birefringent so that the optical fiber responds to the pressure to transversely stress its core. When transversely stressed, the optical grating produces a reflection or transmission from the light beam that has two peaks or minimums in its wavelength spectrum whose spacing and/or spread are indicative of the forces applied to the fiber. One or more detectors sense the reflection or transmissions from the optical grating to produce an output representative of the applied force. Multiple optical gratings and detectors may be employed to simultaneously to measure temperature or the forces at different locations along the fiber. U.S. Pat. No. 5,828,059 is hereby incorporated herein by reference.
Difficulties are encountered in applying the Udd method to measuring pressure in earth formation around a borehole. These difficulties arise mainly from the need to achieve very high resolution to distinguish between the two peaks.
The need to achieve very high resolution to distinguish between the two peaks is addressed by Robert Schroeder in U.S. Pat. No. 5,841,131 (“the Schroeder patent”). The Schroeder patent discloses a fiber optic pressure transducer having enhanced resolution and dynamic range. The Schroeder fiber optic pressure transducer includes a fiber optic core having one or more gratings written onto it, a birefringence structure for enhancing the birefringence of the core, and a structure for converting isotropic pressure forces to anisotropic forces on the fiber core. Schroeder also discloses a spectral demodulation system, including a Fabry-Perot interferometer, for detecting pressure ambient to the fiber optic pressure transducer based on the wavelength and shift of spectral peaks. U.S. Pat. No. 5,841,131 is hereby incorporated herein by reference. A pressure measuring system in accordance with the teachings of Udd and Schroeder is referred to herein below as “the Udd/Schroeder system”.
It is very desirable, in an oilfield-logging context, to use a single pressure-measuring instrument to measure pressure simultaneously at multiple different locations. From this perspective, the Udd/Schroeder system has two related disadvantages as follows. When used to measure pressure simultaneously at twenty or more different locations, it has limited resolution. Alternatively, when configured for a particular resolution, the number of different locations that can be monitored simultaneously is severely limited. Other measuring systems that use multiple transducers to convert physical phenomena to optical wavelengths suffer from similar disadvantages.